Fixed Deposit calculation

Hello friends want to know how banks are calculating FD's. for eg. FD Keep for 399 days @ 7.75% for (senior citizen) rs 1 lakh MIS calculated rs 642/- and same amt kept for 333 days @ :7.90% for (senior citizen) comes to rs. 652. how banks are calculating this interest. I am not able to work it out. can anybody solve this.